Field of the Invention
The following description relates to wireless communication system, and, more particularly, to a device-to-device (D2D) related reporting method performed by a terminal in a wireless communication system and a terminal using the method.
Related Art
In an International Telecommunication Union Radio communication sector (ITU-R), a standardization of International Mobile Telecommunication (IMT)-Advanced being a next mobile communication system after a third generation has been performed. The IMT-Advanced is aimed at supporting an Internet Protocol (IP) based multi-media service with a data transmission rate of 1 Gbps in a stop and low speed moving state and a data transmission rate of 1 Gbps in a high speed moving state.
A 3rd Generation Partnership Project (3GPP) is preparing LTE-Advanced (LTE-A) being an improved one of Long Term Evolution (LTE) based on an OFDMA (Orthogonal Frequency Division Multiple Access)/SC-FDMA (Single Carrier-Frequency Division Multiple Access) transmission scheme as a system standard satisfying requirements of IMT-Advanced. The LTE-A is one important candidate for IMT-Advanced.
In recent years, there is growing interest in a Device-to-Device (D2D) technology performing direct communication between devices. In particular, the D2D is attracting attention as a communication technology for a public safety network. A commercial communication network has been rapidly changed to the LTE but a current public safety network is based on a 2G technology in a collision problem and a cost side with an existing communication standard. Request for the technology clearance and an improved service induces an effort to improve the public safety network.
The public safety network has high service requirements (reliability and security) as compared with a commercial communication network. In particular, when coverage of cellular communication is insufficient or is not used, there is a need for direct signal transmission/reception between devices, that is, a D2D operation.
The D2D operation may be signal transmission/reception between adjacent devices to have various advantages. For example, a D2D terminal may perform data communication with a high transmission rate and low delay. Further, the D2D operation may distribute traffic converged in a base station. If the D2D terminal serves as a relay, the D2D terminal may serve to extend coverage of a base station.
Meanwhile, for a D2D operation, a UE has to establish RRC connection with a network to enter an RRC connection state. However, there are some cases in which the network is not able to aware if the UE performs D2D operation, for example, transmission of a D2D signal, depending on a mode of the D2D operation. Modes of the D2D operation includes a mode in which a resource pool able to transmit a D2D signal is set by a network set, and a resource actually transmitting the D2D signal is determined by the UE. In this mode, it is difficult for the network to exactly know the current state of the D2D operation performed by the UE. Thus, the network may try to maintain the RRC connection state of the UE for the purpose or reliability of the D2D operation. Yet, maintaining the RRC connection state in an unnecessary situation may increase battery consumption of the battery and signaling overhead on the network.